Plasma arc cutting torches are widely used in the cutting, gouging and marking of materials. A plasma arc torch generally includes an electrode, a nozzle having a central exit orifice mounted within a torch body, electrical connections, passages for cooling, and passages for arc control fluids (e.g., plasma gas). Optionally, a swirl ring is employed to control fluid flow patterns in the plasma chamber formed between the electrode and the nozzle. In some torches, a retaining cap can be used to maintain the nozzle and/or swirl ring in the plasma arc torch. In operation, a plasma arc torch produces a plasma arc, which is a constricted jet of mostly ionized gas with high temperature and that can have sufficient momentum to assist with removal of molten metal. A plasma cutting system can include at least one plasma arc torch, a power source for supplying power to the plasma arc torch, and a gas source for supplying a gas (e.g., air) to the plasma arc torch to support various torch operations. In some designs, a compressor is used to compress the gas from the gas source and deliver the compressed gas to the plasma arc torch.
A typical plasma arc torch uses a total of about 240 standard cubic feet per hour (scfh) of air or higher compressed to about 65 pounds per square inch (psi) or higher. This total amount of air is typically directed through various flow paths in the plasma arc torch, such as to the shield, the nozzle, the electrode, and/or to the plasma chamber. FIG. 1 shows the various paths of gas (e.g., air) distribution in a typical plasma arc torch 100, which includes an electrode 102, a plasma chamber 103, a nozzle 104, a swirl ring 106, and a retaining cap 108. The electrode 102 defines a distal end 114 configured to receive an emissive element 116 and a proximal end 115 opposite of the distal end 114. The plasma chamber 103 is defined, at least in part, by the distal end 114 of the electrode 102 and the nozzle 104, which is situated in a spaced relationship from the electrode 102. The nozzle 104 includes a nozzle exit orifice 130. The swirl ring 106 is in fluid communication with the plasma chamber 103 and has at least one radially offset or canted gas distribution hole 118. The retaining cap 108 is securely connected (e.g., threaded) to the nozzle 104. A shield (not shown) can be connected (e.g., threaded) to the retaining cap 108.
In operation, a gas is introduced into the torch 100 through a gas inlet 110 at a flow rate of about 240 scfh or higher, and a gas flow 112 travels toward the distal end 114 of the electrode 102 in a channel between an exterior surface of the swirl ring 106 and an interior surface of the retaining cap 108. As the gas flow 112 passes the gas distribution hole 118 of the swirl ring 106, the flow 112 is divided about equally, approximately 50% of which forms a shield flow 120 and the remaining 50% of which forms a swirl flow 122. The shield flow 120 travels at a flow rate of about 125 scfh or higher in a channel between an exterior surface of the nozzle 104 and an interior surface of the retaining cap 108 eventually exiting the torch 100. The shield flow 120 can cool the nozzle 104, provide stability to the plasma arc generated, and remove dross. The swirl flow 122 travels through the distribution hole 118 and continues toward the plasma chamber 103 in a channel between an exterior surface of the electrode 102 and an interior surface of the nozzle 104. As the swirl flow 122 reaches the plasma chamber 103, the swirl flow 122 divides, about 20% of which (i.e., 10% of the input gas flow 112) forms a plasma chamber flow 124 and the remaining 80% of which (i.e., 40% of the input gas flow 112) forms an electrode vent flow 126. The plasma chamber flow 124 constricts the plasma arc in the plasma chamber 103 and exits the plasma chamber 103 through the nozzle exit orifice 130 at a flow rate of about 19 scfh or higher. In contrast, the electrode vent flow 126 is adapted to travel in a reverse direction from the distal end 114 of the electrode 102 to its proximal end 115 at a flow rate of about 96 scfh or higher and exit the torch 100 through a venting port (not shown) at the proximal end 115 of the electrode 102. The electrode vent flow 126 is adapted to cool the electrode 102 as it traverses the longitudinal length of the electrode 102.
One significant shortcoming associated with a typical plasma arc torch design (e.g., torch 100 of FIG. 1) is that such a torch requires a gas flow rate of about 240 scfh or higher, which represents inefficient use of incoming gas. This also means that a typical plasma arc torch requires a significant amount of compressed gas flow to stabilize the plasma arc and cool various torch components. For example, gas flow rate requirements for a typical plasma arc torch generally start at 4 cubic feet per minute (cfm) and can be as high as 9 cfm.
In addition to shortcomings associated with the high flow rate of the compressed air required to operate a typical plasma arc torch, another shortcoming is the poor quality of the compressed air generated by the compressor of a plasma cutting system. In general, better cut performance is possible if the compressed air delivered to the torch is cool and dry. However, achieving this is a challenge in a plasma cutting system, especially a system with an “on-board” air compressor (i.e., an air compressor integrated in the same housing as the power supply) because such a compressor normally produces hot, humid air. To overcome this limitation, existing designs use one or more after-cooler coils to reduce the temperature of the compressed air, but these coils rely on weak-forced convection to operate, thus generating a low heat transfer coefficient (e.g., about 60 W/m^2-° C.) that produces ineffective cooling.
Furthermore, existing plasma cutting systems have yet to be efficiently adapted for easy, portable usage, especially when the cutting systems have an on-board air compressor. For example, one design requires the air compressor to be powered by fixed input alternating-current (AC) voltage (e.g., 110 VAC or 240 VAC), which limits user options and makes the system difficult to use in field applications. Another design requires a separate power source (other than the source used to power the torch) to power the air compressor, which increases system component cost and reduces portability.
Thus, it is desirable to provide a plasma arc cutting system that has power and gas considerations for operating a plasma arc torch effectively at lower gas flow rate while maintaining about the same gas pressure, thereby enabling lower gas consumption and more efficient gas usage. Additionally, it is desirable to supply a gas to the plasma arc torch that is cool and dry, thereby allowing better torch performance. Moreover, it is desirable to provide a portable plasma cutting system that achieves the desired gas qualities described above, where the portable system can effectively integrate the power supply with the air compressor without introducing inconvenient limitations, such as adding bulky and/or costly components or requiring fixed input voltages.